Holiness is, indeed, Christ in me. But how does Christ "get" in me, as it were? Simple: in communion. In other words, Christ comes to dwell in me by offering Himself to me in and through the Eucharist. So He comes to be in me by my eating His flesh and drinking His blood. This is stunningly simple, very concrete. Where, you might ask, do I get this idea? From our Lord Himself. Two passages from St. John's Gospel will suffice to make my point:
The first passage is from the sixth chapter of the Gospel According to St. John, the part of the chapter known as "The Bread of Life Discourse," which takes place, not coincidentally, the day after Jesus had fed the multitude by multiplying the five loaves and two fish offered by "a boy" who was present among the crowd. In the Gospel narrative the two events are interspersed only by the night-time crossing of the sea of Galilee, during which voyage Jesus joined His disciples, who were crossing by boat, by walking to them across the water, and arrived with them at Capernaum without aid of a boat. Many of the multitude whom Jesus had fed crossed the sea in the morning to look for Him in Capernaum. Upon finding Him Jesus told them why they sought Him- they were looking for Him "not because you saw signs but because you ate the loaves and were filled" (6:26).
This crowd of common people simply wanted another free meal. They were poor. They were hungry. Their "food security" wasn't the best. Jesus did not upbraid, or become angry with them for their pursuit of food. Did He not feed them the previous day? He cared about their material needs. But He wanted to give them, just as he wants to give us, not something, but Someone, better, namely. Himself. He wants to give Himself to us wholly and totally in order that we may realize our destiny, reach our destination, fulfill the very end for which we exist in the first place, to help us grasp that our daily struggle to live has purpose and meaning, which is realized through love, which is the Way to life everlasting.
Jesus exhorted them, "Do not work for food that perishes but for the food that endures for eternal life, which the Son of Man will give you" (6:27). They asked Him, "What can we do to accomplish the works of God? Jesus answered and said to them, 'This is the work of God, that you believe in the one he sent'" (6:28-29). Now we're starting to get somewhere, to this "how Christ gets in me" business.
Still not grasping His meaning, they asked Him what sign He could do so that they could believe He is the One God sent. He replied by telling them that God provided manna for the Israelites during their desert sojourn. Jesus made it clear that it was God, not Moses, who gave them this "bread from heaven" and that the bread God provides "gives life to the world." They wanted the bread that gives life and implored Jesus to give it to them. He said, racheting things up, "I am the bread of life; whoever comes to me will never hunger, and whoever believes in me will never thirst... I am the bread that came down from heaven" (6:35. 41).
When he said this the crowd was incredulous and asked,, "Is this not Jesus, the son of Joseph? Do we not know his father and mother? Then how can he say, 'I have come down from heaven'?" Jesus responded by telling them to stop their murmuring (6:42-23).
I encourage you to read the whole thing for yourself, but here is what the Lord says that matters: "Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is true food, and my blood is true drink. Whoever eats my flesh and drinks my blood remains in me and I in him. Just as the living Father sent me and I have life because of the Father, so also the one who feeds on me will have life because of me. This is the bread that came down from heaven. Unlike your ancestors who ate and still died, whoever eats this bread will live forever" (6:54-58).
Proof that Jesus was not speaking figuratively, but quite literally, is that He asked His disciples if His words "shock" them (6:61). Further proof is that many who had followed Him to that point left (6:66- I certainly don't want to attach any undue or superstitious significance to it, but I find it a bit startling that this is verse 6-6-6. Of course, the chapter and verse numbering system are NOT part of the inspired text. I just find it a weird coincidence). It's safe to assume that they didn't leave because they misunderstood what He said. After all, He didn't run after them yelling, "No, wait a minute. You don't understand!" Rather, He turned to the Twelve and asked them, "Do you also want to leave?" Peter, speaking on behalf of the Twelve, replied, "Master, to whom shall we go? You have the words of eternal life. We have come to believe and are convinced that you are the Holy One of God" (6:68-69).
The second passage is from John 17- from Jesus' "High Priestly Prayer." He prayed that all who believe in Him will be one as He is one with the Father (17:11). How is the oneness achieved? We become "one" as the Father is in Jesus and Jesus comes to be in us so that, in the words of our Lord, "that the world may believe that you sent me." Jesus gives us the glory that the Father gave Him in order "that they may be one, as we are one, I in them and you in me, that they may be brought to perfection as one, that the world may know that you sent me, and that you loved them even as you loved me" (17:21-23). The Father is in Jesus and Jesus comes to be in us by the power of the Holy Spirit.
It is the Holy Spirit who, as the epiclesis in each Eucharistic prayer reminds us, transforms the bread and wine into Jesus Christ, Body, Blood, Soul, and Divinity, making it the heavenly bread that gives us life and through us gives life to the world. This is central to Pope Francis' call for us to be missionary disciples. Without Jesus giving Himself to us in the Eucharist we can do nothing.
There is nobody who should be more humble about Christ present to us in the Eucharist than priests, through whom Christ deigns to make Himself present to us (in order to be "in" us) in this way. If that doesn't cause a person to fall on his face in humility and unworthiness, I don't know what would.